This invention generally relates to optical motion encoders, such as a shaft encoder or a strip encoder. In particular, the present invention improves and simplifies the structural design of optical motion encoders for translation of movement information such as the direction and increment of rotational or translational movement of an object into output signals indicative of the movement.
Prior art optical motion encoders of the present kind translate rotary or linear motion information into a digital output.
Exemplary rotary and linear optical motion encoders are known from U.S. Pat. No. 4,691,101. The rotary encoder, also known as a shaft angle encoder, includes a light source for emitting a light beam, an encoder wheel for modulating the light beam in response to shaft rotation, and a detector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the detectors.
As the light is modulated in response to shaft rotation, each electrical signal from the detector assembly produces a wave form. The position of the shaft determines the position of each signal on its particular wave form, that is, the phase of each signal. Thus, the electrical signals from the detectors can be used to indicate shaft rotation. Two or more properly out-of-phase signals, from separate detectors, can be used to indicate both direction and magnitude of rotation.
Typical shaft angle encoders have an opaque wheel with a plurality of transparent windows through which illumination passes. In the simplest embodiment, a light source is provided adjacent to one face of the wheel. A photodetector is placed on the opposite side of the wheel so that as the wheel rotates, light from the emitter is alternately passed through a window to the photodetector or occulted by an opaque area of the wheel. In a linear motion optical encoder, the encoder wheel is replaced by a encoder strip which is provided with transmissive and non-transmissive areas for modulating light from an emitter.
There may be a number of drawbacks to such arrangements, particularly when relatively low-cost encoders are desired. These drawbacks stem from having the light emitter on one side of the wheel/strip and the light detectors on the opposite side of the wheel/strip. This means that there are two substrates to which electrical connections must be made. Having two substrates complicates the positioning and alignment of components to obtain a desired precision. Each of these results in difficulties in the assembling process which in turn adds significantly to the cost of the encoder.
Further, to obtain precision and high contrast, it is commonly necessary to employ collimating optics for the light source. This adds an element of complexity and difficulty in assembly alignment and also enlarges the size of the encoder. In the modern world, it is almost universal that small size is desirable.
It is, therefore, desirable to produce an optical encoder where the light emitters and photodetectors can be mounted on a single substrate.
U.S. Pat. No. 4,952,799, discloses a reflective shaft angle encoder in which the light emitters and photodetectors are mounted on a single substrate. This is achieved by employing an optical shaft angle encoder having a code wheel with alternating reflective and non-reflective areas in a circumferential path on one face of the wheel. A light emitting diode is spaced apart from the reflective areas on the wheel. As the wheel rotates, images of the reflective and non-reflective areas are projected to the photodetectors which in turn generate signals indicative of the motion of the wheel.
However, reflective optical encoder arrangements of the type disclosed in U.S. Pat. No. 4,952,799 are very sensitive to the relative positions of the code wheel and the photodetectors. The image of reflective and non-reflective areas projected to the photodetectors varies greatly with the orientation of the code wheel and the distance the code wheel is spaced apart from the photodetectors. Consequently, having a reflective code wheel complicates the positioning and alignment of the encoder components which adds significant cost to the encoder. Furthermore, precision and high contrast are also compromised by the lack of light source collimation in the reflective optical encoder arrangements.
It is an object of the present invention to provide an optical motion encoder that has a simplified design to simplify the assembling process and to at least reduce sensitivity for misalignment of the optical components used.
Another object of the present invention is to provide an optical motion encoder that allows for flexibility in designing absolute and incremental light encoding pattern with various resolution.
It is a further object of the present invention to provide an optical motion encoder of the present kind that allows in a simple way to include a visual output of the light source.
An optical motion encoder for translation of movement information into a digital output that embodies the principles of the present invention includes the integration of a collimating reflector surface and a code patterned planar surface into a single optical member. The optical member moves with respect to a reference axis and has reflecting surface for reflecting light rays received from a point light source on the reference axis in a direction which is substantially parallel with the reference axis. Preferably, the reflecting surface is an parabolic surface symmetrical about the reference axis.
The optical member further includes a planar surface which is perpendicular to the reference axis for receiving the parallel light rays reflected from the reflector surface and is provided with a code pattern for modulating the light rays passing through the planar surface when the optical member is moving. The modulated light is detected by a light sensor arrangement which is preferably aligned in parallel with the reference axis and the modulated light rays. The light sensor is configured to detect light signals in a desired way to provide an output signal indicative of the motion of the optical member. The output signal is preferably a digital output signal with one or more channels.
In a preferable embodiment of the present invention, the optical motion encoder is designed as an optical shaft encoder processing a rotational movement around the reference axis, wherein the reflecting surface is a parabolic concave surface of an axially symmetric paraboloid of revolution having the reference axis as its axis of symmetry.
In a further preferred embodiment, the optical member is extended at the apex of the reflecting surface by a translucent shaft extending along the reference axis. Such a shaft, which is aligned with the light source, serves in this way as visual output of the light source for visual feedback. If the motion encoder according to the invention is a shaft encoder, said translucent shaft acts as a light pipe and can also be used as the shaft of the shaft encoder for mounting the encoder to the rotating object the rotary movement of which should be detected.
Due to the structural design of the motion decoder of the invention, the reflector and the encoding component can be assembled in a simple way thereby realizing the one-part optical member of the invention. The optical member can even be constructed as a translucent body of optical glass or plastics in the shape of a half of a paraboloid having its parabolic surface coated with a reflective coating to form the reflecting surface and having the code pattern printed or etched on its planar base surface.